DIFFERENTIATION OF NERVOUS STRUCTURE 115 
tion modifies the structural substratum, and this in 
turn modifies further function and so on. If the effect 
of functional activity on the structural substratum is 
ever completely reversible it is only or chiefly in the 
fully developed organism where a dynamic equilibrium 
between structure and function is attained or approached. 
Development represents the progress from a disturbed 
protoplasmic or cellular equilibrium to an organismic 
equilibrium, or, as in physiologically isolated regions or 
physically isolated pieces, the progress from a disturbed 
organismic equilibrium to a new equilibrium, and the 
metabolic or physiological gradients constitute the 
framework or ground plan on which this progress takes 
place. 
The primary germ layers represent the primary 
regional differentiation with respect to the gradient 
pattern. They result from the differences in rate of 
metabolism at different levels of the gradient and con- 
stitute the basis for a chemical or transportative relation 
which arises sooner or later, when ectoderm grows at the 
expense of the yolk in the entoderm, or of food digested 
by the entoderm. This transportative relation between 
ectoderm and entoderm is, however, secondary and the 
consequence of the differences in rate of activity between 
them. The nervous system, on the other hand, repre- 
sents the primary differentiation with respect to the 
functions of excitation and transmission of the gradients, 
and as the gradients represent the primary integrating 
factor in the development of the organism, the nervous 
system as the differentiated mechanism of this primary 
integrating function necessarily becomes the chief organ 
of integration. 
